In depleted uranium (DU) alloy projectiles, adiabatic shear (AS) banding serves as a self-sharpening mechanism. The onset of plastic instability, as thermal softening overcomes internal work-hardening mechanisms, results in these localizations. The AS bands provide a failure mechanism by which the DU projectile can discard deforming material at its head, reducing or preventing the build-up of a large mushroomed head, and allow the DU projectile to efficiently burrow through armor.
Efforts to develop alternatives to DU for projectile applications have focused on imparting a similar deformation behavior in tungsten heavy alloys (WHAs). Conventional WHAs are two-phase composites of nearly unalloyed tungsten particles embedded in a nickel (Ni) alloy matrix, produced by liquid-phase sintering of metal powders. Because the W phase itself is very resistant to AS localization, primary focus has been placed on replacing the Ni alloy matrix with an alloy matrix having a greater susceptibility to AS failure that may illustratively include alloys formed of at least one or more pyrophoric components such as uranium (U), titanium (Ti), zirconium (Zr), or hafnium (Hf).
The recent development of Zr- or Hf-based bulk metallic glass alloys (BMGs), of compositions with much lower critical cooling rates and thus castable in thicker sections, makes them interesting candidates for projectile material applications. Specifically, BMGs generally possess very high elastic strain limits (2% to 3%) and therefore very high yield strengths (between 1.6 GPa and 2.0 GPa). Beyond their elastic limits, however, BMGs do not strain harden, and plastic deformation is immediately localized into shear bands. Shear bands thus serve as a BMG's sole mechanism of plastic flow, under quasi-static as well as dynamic loads. The localization is generally modeled as resulting from a reduction in local viscosity, associated with an increase in “free volume” as atoms move within the amorphous structure, but there is not a universally agreed-upon explanation for this behavior. At higher strain rates, the additional thermal-softening component leads to an earlier failure along one of the first shear bands, reducing the net accumulation of the plastic deformation.
The differences in the shear banding behaviors of DU alloys versus BMGs or W-based BMG matrix composites (W-BMG) leave many questions about the potential of BMG composites to ballistic applications. However, subscale ballistic tests of monolithic Hf-based BMGs and W-BMGs have shown that the post-impact evaluation of residual projectile and erosion products recovered in a steel target plate can be inherently difficult. Because of their low crystallization temperatures, the heat generated by friction and by the plastic work done to the steel target in opening the penetration cavity, causes devitrification (i.e., crystallization or loss of the amorphous substructure) of the “as-sheared” BMG matrix, thereby erasing the evidence of any shearing behavior.
Projectile cores are typically fired from a laboratory gun system having a 10-ft (3-m) long, thick walled, smooth bore gun; a 38-mm bore diameter; and a fitted 40-mm breech system as generally illustrated in prior art FIG. 1. This system allows for very controlled launch velocities and relatively gentle launch accelerations. The projectile is assembled in a four piece plastic sabot and push launched from behind by a 0.3-in (7-mm) thick steel disk set in a polypropulux obturator (not shown). The obturator traps the gases behind the launch package and accelerates the projectile down the barrel. The barrel is located approximately 2–3 m from the target impact. This short flight distance eliminates the need for any flight stabilization via fins or spin.
Still referring to prior art FIG. 1, a conventional experimental facility may be instrumented with radiographic equipment to capture the terminal ballistic event. In this manner, two images of the projectile can be captured prior to impact, in both the horizontal and vertical planes. Reference fiducial wires on X-ray radiographs R are used to determine the orientation of the projectile. By measuring the positions of the projectile images relative to the fiducial wires and the preset time delays between successive images, all pertinent information relative to speed and orientation is determined. The radiographs also serve to record the images of the residual projectile and target debris.
Typically, subscale projectile materials are launched at modest velocities and impact a hardened steel target, placed at the rear of the test stand. As the projectile impacts the target surface, extreme pressures cause it to fail, leading to backward jets of finely divided fragments. While usually unpracticed, if desired, fragments of the erosion products from the projectile could be captured in a containment box.
For conventional projectile materials (e.g., WHAs) this technique would be quite sufficient. However, for BMG-based projectile materials this method is inadequate. Particularly, a BMG material consisting of about 65 to 70% by weight Zr or Hf, both of which are pyrophoric, would instantaneously burn up or chemically react with the surrounding air. Moreover, fragments of the residual projectile or its erosion products that remain in contact with the hot steel of the target plate would readily devitrify. Thus, a need exists for a ballistic testing recovery apparatus and method that will allow for post-impact evaluation of the “as-ejected” characteristics of residual projectile and erosion products which may be susceptible to pyrophoric degradation.